1. Field of the Invention
The present invention relates to an amplifier, and more particularly to an amplifier adapted for use in a circuit for controlling signals to be supplied to a recording head.
2. Related Background Art
At first there will be explained conventional magnetic recording apparatus employing such amplifier while making reference to FIGS. 3A and 3B, wherein shown are an input signal .nu..sub.01 ; a buffer amplifier a0; a switch SWa; driving amplifiers b0, c0; terminals CH1, CH2 to be onnected to magnetic heads constituting loads; and a capacitor Ca.
In case of recording a first field of an image signal in the circuit shown in FIG. 3A, the switch SWa is so connected as to supply the signal from the buffer amplifier a0 to the driving amplifier b0, so that the input signal .nu..sub.01 is supplied to the recording head connected to the terminal CH1. In recording a second field, the switch SWa is so shifted as to supply the signal from the buffer amplifier a0 to the driving amplifier c0, whereby the input signal .nu..sub.01 is supplied to the recording head connected to the terminal CH2.
In the circuit shown in FIG. 3B, the capacitor Ca is inserted between the buffer amplifier a0 and the switch SWa, but the switching operation thereof is conducted in the identical manner as in FIG. 3A.
In the conventional circuit shown in FIG. 3A, the input terminal of the driving amplifier b0 or c0 is either connected to the buffer amplifier a0 or left open. Consequently the current supplied to the driving amplifier is different and causes a change in the bias according to the positin of the switch SWa.
As the result, when the magnetic head connected to the terminal CH1 is switched to that connected to the terminal CH2, there will result a transient current because of a change in the bias, so that the initial portion of the second field may not be properly recorded.
In the circuit shown in FIG. 3B, such change in bias does not occur since the buffer amplifier a0 is not connected directly, in an IC circuit, to the driving amplifier b0 or c0.
However, the capacitor Ca is charged by the potential difference across said capacitor, and the accumulated charge flows into a circuit to which the switch SWa is connected, when it is shifted. As the result a transident current is generated when the magnetic head is shifted for recording the second field, so that the initial portion thereof may not be properly recorded.
Also the use of the above-explained analog switch SWa gives rise to the following drawbacks:
(1) Such analog switch generally shows unsatisfactory frequency response for frequencies exceeding several MHz; and
(2) A load circuit connected to the driving amplifier, being a relatively large load to the buffer amplifier, shows a deteriorated output characteristic, by the influence of the contact resistance of the analog switch.
It is also considered, as shown in FIG. 3C, to utilize the heads connected to the terminals CH1, CH2 for both recording and erasing, and to select the signals to be supplied to said head by means of a switch SWb.
In the circuit shown in FIG. 3C, the recording signal .nu..sub.s and the erasing signal .nu..sub.e are selected by the switch SWb.
Said switch SWb may be eliminated in a circuit shown in FIG. 3D, wherein the recording signal .nu..sub.s and the erasing signal .nu..sub.e are respectively amplifier by buffer amplifier d0, e0 and are added through resistors R1, R2. Thus, said signals are supplied to a buffer amplifier f0 through the permanently connected adding circuit. In the recording operation the recording signal .nu..sub.s alone is supplied, and, in the erasing operation the erasing signal .nu..sub.e alone is supplied. The input signal is supplied to the the terminal CH1 or CH2 through the switch SWb and a driving amplifier g0 or h0.
However the conventional circuits shown in FIGS. 3C and 3D are associated with the following drawbacks.
The circuit shown in FIG. 3C is associated with the drawbacks that:
(1) the frequency characteristic of the recording signal .nu..sub.s deteriorates in passing through the switch SWb; and
(2) distortion will result due to the presence of the switch SWb.
The the circuit shown in FIG. 3D includes a drawback that the S/N ratio of the recording signal .nu..sub.s deteriorates by the noise introduced from the input circuit of the erasing signal .nu..sub.e.
Also FIGS. 3E to 3G show examples of an amplifier for adding and amplifying plural input signals, wherein shown are input signals .nu..sub.1, .nu..sub.2, .nu..sub.3 ; transistors Q1-Q3, Qd, Qe; capacitors C1-C3, Ca, Cb, Cc: resistors R1-R9, Ra, Rb, Rc, Rd, Re, Rg, Rh; and a driving circuit 1, for example for a magnetic recording system.
In the circuit shown in FIG. 3E, the input signal .nu..sub.1 is supplied, through the capacitor C1, to the base of the transistor Q1 and to the resistors R1, R2. The other end of the resistor R1 is connected to a constant voltage source Vcc, and the other end of the resistor R2 is grounded. The collector of said transistor Q1 is connected to the constant voltage source Vcc, and the emitter thereof is connected to the resistor R3 and to the resistor Ra through the capacitor Ca. The other end of said resistor R3 is grounded. Consequently the signal .nu..sub.1 is transmitted through a buffer amplifier consisting of the transistor Q1 and the resistors R1, R2 and R3, and is released from the emitter of the transistor Q1 to the resistor Ra through the capacitor Ca. Similarly the signal .nu..sub.2 is supplied through the capacitor C2, transmitted through a buffer amplifier consisting of the transistor Q2 and the resistors R4, R5 and R6, and released from the emitter of said transistor Q2 to the resistor Rb through the capacitor Cb. Likewise the signal .nu..sub.3 is transmitted through a buffer amplifier consisting of the transistor Q3 and the resistors R7, R8 and R9, and is relaased from the emitter of said transistor Q3 to the resistor Rc through the capacitor Cc. The other ends of three output resistors Ra, Rb, Rc are mutually connected and are further connected to the base of the transistor Qd.
In the above-explained circuit, three input signals .nu..sub.1, .nu..sub.2, .nu..sub.3 are respectively amplified by buffer amplifiers composed of the transistors Q1, Q2, Q3 and are added at the base of the transistor Qd through the resistors Ra, Rb, Rc.
The output from the emitter of the transistor Qd is supplied to the driving circuit 1 for driving a recording head. It is assumed that the capacitors Ca, Cb, Cc are so selected as to satisfactorily transmit the frequency ranges of the input signals .nu..sub.1, .nu..sub.2 and .nu..sub.3.
In the circuit shown in FIG. 3F, three input signals .nu..sub.1, .nu..sub.2, .nu..sub.3 are likewise amplified respectively by buffer amplifiers composed of the transistors Q1, Q2, Q3 and are transmitted through the resistors Ra, Rb, Rc, but they are mutually added at the emitter of the transistor Qe.
The output from the collector of said transistor Qe is supplied to the driving circuit 1. As explained above, the capacitors Ca, Cb, Cc are so selected as to satisfactorily transmit the frequency ranges of the input signals .nu..sub.1, .nu..sub.2 and .nu..sub.3.
In the circuit shown in FIG. 3G, three input signals .nu..sub.1, .nu..sub.2 and .nu..sub.3 are amplified by three emitter-grounded amplifiers composed of three transistors Q1, Q2 and Q3 having a common load Rh at the emitters thereof, then mutually added at said common load Rh and are supplied to the driving circuit 1.
However the conventional circuits explained above are associated with various drawbacks.
The circuit shown in FIG. 3E is associated with the drawbacks that:
(1) the gains for the input signals .nu..sub.1, .nu..sub.2, .nu..sub.3 are determined respectively as:
Ra/(Ra+Rb+Rc), Rb/(Ra+Rb+Rc), Rc/(Ra+Rb+Rc) and cannot be independently regulated;
(2) a separate amplifying circuit is required for obtaining a desired gain, since the gain in this circuit is less than 1 for the input signal .nu..sub.1, .nu..sub.2 or .nu..sub.3.
The circuit shown in FIG. 3F is associated with the drawbacks that:
(1) though the gains for the input signals .nu..sub.1, .nu..sub.2, .nu..sub.3 are respectively determined by Rg/Ra, Rg/Rb and Rg/Rc and are therefore independently regulable, the frequency characteristic tend to deteriorate when the gain is increased, since the output is obtained from the collector of the transistor Qe; and
(2) since the output is obtained from the collectr of the transistor Qe, the output characteristic is deteriorated when a large load such as a driving circuit for a recording head is connected.
The circuit shown in FIG. 3G is associated with the drawbacks that:
(1) though the output gains for the input signals .nu..sub.1, .nu..sub.2, .nu..sub.3 are respectively defined by Rh/R3, Rh/R6 and Rh/R9 and, therefore, can be regulated independently, the frequency characteristic is insufficient because of the use of the emitter-grounded amplifiers; and
(2) since the output is obtained from the collector of the transistor, the output characteristic deteriorates when a large load such as a driving circuit for a recording head is connected.